In a so-called electronic watch using a crystal oscillator or the like as a time base, as shown in FIG. 1, a power supply section 10 is made up of a small-sized dynamo 20 and a secondary power supply 30, and a stepping motor 40 is driven by power supplied from the power supply section 10. A watch wheel train 50 is operatively connected to a motor rotor 42 of the stepping motor 40 so that, for example, a second hand 161 attached to a second wheel 52 is intermittently rotated in steps of 6 for each second.
On the other hand, the small-sized dynamo 20 comprises a dynamo rotor 21 rotated by torque transmitted to it, a dynamo stator 22 disposed in surrounding relation to the dynamo rotor 21, and a dynamo coil 23 wound over a magnetic core 24 making up a magnetic circuit in cooperation with the dynamo stator 22 and the dynamo rotor 21. A dynamo wheel train 60 for transmitting rotation of an oscillating weight 25 while speeding up the rotation is operatively connected to the dynamo rotor 21.
In the field of electronic watches with hands, there is a strong demand for a reduction in thickness even in the above-mentioned type having a small-sized dynamo. However, such a demand for a reduction in thickness cannot be satisfied simply by reducing the size or thickness of various parts, e.g., the oscillating weight 25 as one component of the small-sized dynamo. For example, if the thickness of the oscillating weight 25 is reduced, weight unbalance of the oscillating weight 25 in the angular direction would be diminished and the oscillating-weight 25 would be hard to rotate at a high speed. Also, because necessary parts are mounted on a circuit board 31 constituting a circuit section, the circuit section cannot be further reduced in size and thickness. If it is nonetheless attempted to reduce a space in which the circuit section is installed, there would occur a risk that electronic parts and so forth may interfere with gears of the dynamo wheel train 60 and the watch wheel train 50.
A rotational shaft of the dynamo rotor 21 and a rotational shaft of the dynamo wheel train 60 are each often supported by a small and simple bearing formed of a hole jewel. In the bearing structure using a hole jewel, however, a lubricant applied to the rotational shaft tends to scatter to the surroundings upon rotation of the rotational shaft. If the scattered lubricant adheres to the watch wheel train 50, the lubricant may cause abnormal motion in driving the hands, such as stop or delay of any of gears, due to its viscosity. This raises a problem in conventional electronic watches with hands in that the parts cannot be arranged in closer relation and hence the thickness of the watch cannot be reduced.
Further, in the conventional electronic watches with hands, as shown in FIG. 11, one of the gears of the dynamo wheel train which tends to be easily subject to lateral pressure, such as a dynamo rotor transmitting wheel 62A (see FIG. 1), is sometimes supported at its rotational shaft 20A by a ball bearing 28A. The ball bearing 28A comprises a plurality of balls 281A arranged around the rotational shaft 620A of the dynamo rotor transmitting wheel 62A, a ring-shaped frame piece 282A holding the balls 281A, and a retainer piece 283A positioned adjacent the frame piece 282A to cooperate with it to prevent the balls 281A from slipping off. The balls 281A are held in contact with the rotational shaft 620A to restrict a lateral inclination of the rotational shaft 620A. Also, the rotational shaft 620A has a stepped portion 626A formed around it, and the stepped portion 626A abuts against the retainer piece 283A to restrict the position of the rotational shaft 620A in the axial direction.
However, the bearing structure shown in FIG. 11 has a problem that large friction resistance generates between the stepped portion 626A and the retainer piece 283A when the rotational shaft 620A is rotated. Generation of large friction resistance means that wasteful excessive force is required to rotate the rotational shaft 620A, and that the stepped portion 626A or the retainer piece 283A is severely worn away. Thus, there is a need for a novel bearing structure capable of solving the above-stated problems. However, even a bearing structure which has succeeded in solving the above-stated problems cannot be practically adopted if it requires a larger space, because such a bearing structure prevents a reduction in thickness of electronic watches with hands.
In view of the problems stated above, an object of the present invention is to provide a construction of an electronic watch with a built-in dynamo, which can improve structures of parts themselves arranged inside the watch and layout of the parts, and can reduce a total thickness of the electronic watch.